1. Field of the Invention
This invention relates to a rotary rock bit having a sealed, pressure-equalization lubrication system, and more particularly, to such bit lubrication system and structure wherein the lubricant reservoir is disposed within the axially extending journal and immediately adjacent the rotary cutter bearing structure to minimize the length of the lubricant flow path, and thereby decrease the reaction time of the pressure equalization between the reservoir and the bearing seal.
2. Description of the Prior Art
Sealed, pressure-equalizing lubrication systems for rotary rock bits are well known as shown, for example, by U.S. Pat. No. 4,199,856 of common assignee with the present invention. The structure shown therein is generally typical of the presently commercially successful rotary rock bits, and it is seen to include a diaphragm-sealed lubricant reservoir disposed in the upper portion of each arm of the bit on which a rotary cutter is mounted. The diaphragm is in fluid communication with the exterior of the bit to be responsive to the external fluid pressure.
A lubricant flow path leads through the arm from the lubricant reservoir to various outlets in or adjacent the journal or pin supporting the bearing structure of the rotary mounted cutter. The back face of the rotary cutter (i.e. the face adjacent the arm) contains an elastomeric seal for sealing the bearing structure from the external environment of the bit.
In operation, the external environment for the bit is typically the abrasive drilling mud and cuttings circulating at high velocity and pressures as the bit is rotated downhole. Under static conditions, the flexible diaphragm of the lubricant reservoir maintains the internal pressure of the lubricant substantially equal to the external pressure so there is essentially no pressure differential across the seal or across the diaphragm. Thus, under such conditions, there is no appreciable tendency for the drilling mud to leak into the bearing and lubrication system, nor for the lubricant to leak out.
However, in practice, it has been found that the dynamics of operation, such as the occasional bouncing of the bit on the bottom of the borehole as it rotates, causes localized pressure surges in the external fluid that develop, although maybe for only relatively short duration, pressure spikes or peaks that may also be rather localized at certain locations on the bit. Because the lubricant filling the reservoir and flow paths is relatively viscous, and the lubricant path between the diaphragm and the elastomeric seal quite long, such pressure surges cannot be immediately pressure compensated within the lubricant system, resulting in, on occasion, substantial pressure differential existing across the seal. These pressure differentials can in turn force the seal to assume a high rubbing or frictional engagement causing rapid, if not immediate failure. Further, depending upon whether the external pressure is relatively high or relatively low with respect to the internal lubricant pressure, the abrasive mud can be forced past the seal into the bearing cavity to attack the bearings and also abrade the rubber face of the seal, or the lubricant can be forced out of the bearing housing, which in time will deplete the finite supply of lubricant contained within the reservoir. Further, the long paths and high viscosity of the lubricant can, on occasion, contribute to insufficient lubricant being forced into the bearings to replace lost or expelled lubricant. Any of the above generally leads to premature failure of the bearing and bit.